MENLO PARK, Calif. (USA TODAY) - Google unveiled a big revamp of its search engine Thursday that effects 90 percent of the search results served up worldwide by the Internet giant.

Called Hummingbird internally, the change to Google's main search algorithm kicked in about a month ago, but was not disclosed by Google until Thursday at an event in Silicon Valley marking the company's 15 year anniversary.

"It is really big," said Google search executive Amit Singhal at the event.

The new algorithm makes search results more relevant and useful, especially when users ask more complex questions - something that has been happening a lot more in recent years, Singhal explained.